1. Field of the Invention
The invention relates to the field of amplification of data signals in a network such as Ethernet signals in a local area network using standard telephone lines (twisted pairs).
2. Prior Art
Standard telephone lines (twisted pairs) are commonly used in local area networks (LANs). These lines, while not ideal for high frequency signals, are used because they are found in virtually every commercial building.
The typical data rate of signals transmitted in a LAN is substantially higher than the frequencies of the audio signals originally intended for transmission on telephone lines. Consequently, the electrical characteristics of the twisted pairs are not ideal for higher frequency signals. These lines exhibit higher attenuation as the frequency rises. For example, a hundred feet of telephone line might have a loss of 2 dB at 5 MHz and a loss of 3 dB at 10 MHz. This frequency-attenuation characteristic causes a propagated signal having a spectrum of energy to undergo distortion in time and amplitude. Often, a signal quantitized in time intervals suffers from intersymbol interference due to this frequency dependency of attenuation. Accordingly, it is not sufficient to just amplify the signal transmitted over such a line with an amplifier having a "flat" response, but rather correction must also be made for the de-emphasis of the higher frequency.
Consider the problem of amplifying a high frequency signal transmitted over 1000 feet of telephone line. If the frequency response of the line is measured, an amplifier can be readily built to amplify the signal. The problem is, however, that each line requires a unique amplifier. Line characteristics can be quantitized to increments (e.g., 300 feet) and a de-emphasis assumed thereby allowing the selection of an appropriate amplifier as a function of line length. But, for example, if an amplifier designed for a 1000 feet run is used at the end of a 500 feet run, a serious problem can result. Specifically, the amplifier will over-emphasize some higher frequencies of the data signal. This will result in distortion which can change one binary state to another in a detected signal.
As will be seen, the present invention solves this problem by providing an amplifier which automatically compensates for line length.